TECHNICAL FIELD
[0001] The present invention relates to the conversion of solar energy into electricity
and of the photodetection of light generally. In particular, the present invention
relates to the absorption of light energy in a thin metallic layer and its conversion
through irreversible energy selective extraction into an adjacent semiconductor region.
BACKGROUND ART
[0002] This invention is related to the field of infra-red Schottky barrier photodetectors,
in which a metallic light absorbing layer is placed next to a semiconductor layer,
forming an electrically rectifying junction which can be used to detect incident light.
The present invention uses this technology in, and is also related to, the field of
next generation photovoltaics in which the incoming light is efficiently converted
into electrical energy.
[0003] The most conventional form of a Schottky barrier photodetector is shown in FIG. 1,
consisting of a thin metallic film (101) adjacent to a semiconductor layer (102),
with a reflective under layer (103) and an optional anti-reflective coating (ARC)
layer (100). The operation of this class of devices relies on the photo-generation
of ballistic carriers in the thin metal layer by an incident light beam (104), followed
by irreversible extraction into the semiconductor layer prior to the electrons thermalizing
in the metal layer. Therefore, to function, this device necessarily has a metallic
layer that is thinner than the electron mean free path in the metal, such that the
electrons are still ballistic when they reach the metal-semiconductor interface and
are irreversibly extracted from the metallic layer to the semiconductor.
[0004] The thermalisation of electrons in a metal layer can be split into two stages, which
are schematically illustrated in FIG. 2, showing the energy profiles of electrons
in a metal at different times (200-202) following photoexcitation. Electrons are excited
(203) to energies in excess of the fermi energy
Ef (205) by absorbing light, through Drude absorption, and initially create a profile
(204). At this time (200) the electrons in the population (204) are ballistic and
have not suffered any interaction events with either the lattice or other electrons.
After a time of 0.1-1ps (201) the electrons will have interacted with each other and
the electron population in excess of the fermi energy (206) is described as hot, because
the electrons have interacted with each other and so are thermalized amongst themselves,
but have not interacted with the lattice and so still have a temperature in excess
of the lattice temperature. The interaction with the lattice takes place on a timescale
of 1-10ps and results in an equilibrium electron distribution (202) which has the
same temperature as the lattice. It is necessary to extract carriers before this time
in order to have a successful photodetector or photovoltaic cell.
[0005] A schematic illustration of the operation of a conventional Schottky barrier photodetector
is shown in FIG. 3. A metallic layer deposited on an n-doped semiconductor layer normally
gives rise to a rectifying Schottky barrier in the conduction band (300) of the device.
Illumination with light (301) warms the electron population (302) to an electron temperature
(T
e) which is in excess of the equilibrium lattice temperature. These hot carriers are
able to irreversibly travel over the Schottky barrier (303) and generate a current
in the device. This current is used to determine the intensity of light illuminating
the device in a photodetector.
[0006] Early devices were based on thin films of metal which were so thin that they were
partially transparent in order to ensure they were thinner than the mean free path
of electrons in the metal. These devices were somewhat limited by low efficiency,
and all related devices are specifically aimed at photodetection rather than photovoltaic
conversion, as ballistic extraction results in the loss of low energy incident photons.
This was somewhat improved in
US4394571 (Jurisson, issued July 19, 1983), with the introduction of a quarter wavelength cavity to enhance metallic absorption.
However, the device still relied on immediate ballistic extraction of photoexcited
carriers. Further improvements have focused on wave-guiding (
WO 2011/112406, Patel et al., published September 15, 2011), and improved absorption in metal layers has been shown through the use of resonant
absorption (
US 8536781, Lee at al., issued September 17, 2013) and plasmonic absorption (
US 5685919, Saito et al., issued November 11, 1997).
[0007] In addition to prior art in infra-red photodetection, the present invention may be
compared to hot carrier photovoltaic cells. Hot carrier photovoltaic cells operate
by extracting carriers after they have thermalized among themselves, but before they
have thermalized with the lattice (i.e. at stage (201) in FIG. 2). These devices have
been the subject of investigation since then 1980s and one based on
US8975618 (Dimmock et al., issued March 10, 2015) has recently been realised. However, all such cells rely on absorption in a thin
semiconductor layer and thus are limited in efficiency due to low total light absorption.
SUMMARY OF THE INVENTION
[0009] None of the background art has proposed a hot carrier photovoltaic cell using absorption
in a metallic layer with extraction of electrons in such a device by any means other
than through thermionic emission over a Schottky barrier. This invention proposes
a new optoelectronic device structure to enhance the absorption of light in very thin
(>30nm) metal layers and extract photoexcited carriers from that metal region.
[0010] Conventional devices of this kind are commonly used in photodetection, which is often
operated in forward bias (304) but can be used to generate power if operated in reverse
bias (305) in a photovoltaic cell configuration. However, the low device efficiency
means this device has not seen use in photovoltaics. The present invention provides
an improvement to the device efficiency which could allow this device not only to
operate as an efficient photodetector, but also as a photovoltaic cell.
[0011] The novel structure includes a thin metallic layer with an anti-reflective dielectric
coating to air and a series of semiconductor layers with a highly reflective back
coating beneath. The layer structure is tuned such that greater than 90% absorption
of incident light between 400-1000nm is possible in the thin metallic layer. This
incident light absorption heats the electron population in the metal, which is then
extracted into the semiconductor layers energy selectively.
[0012] Aspects of the invention include a semiconductor device having a layered structure.
In exemplary embodiments, the semiconductor device may include a metallic layer of
thickness 1-100nm, with a thickness optimised to absorb light in a wavelength range
of operation. The device further may include an adjacent semiconductor layer additionally
adjacent to an ohmic electrical contact, wherein the interface between the metallic
layer and the semiconductor layer is electrically rectifying and energy selective.
The device further may include a reflective back surface positioned opposite to the
semiconductor layer relative to incident light providing broadband reflection in the
wavelength range of operation. The semiconductor layer may include a quantum well
adjacent to the metallic layer, wherein the energy selectivity is provided by the
quantum well allowing charge carrier tunneling from the metallic layer. The device
further may include an additional anti-reflection dielectric layer deposited on the
metallic layer that is configured to minimise reflection of light in the wavelength
range of operation.
[0013] The advantages of a device in accordance with the present invention include the following:
- The use of a metallic absorber; and
- Energy selective extraction from the metallic absorber.
[0014] To the accomplishment of the foregoing and related ends, the invention, then, comprises
the features hereinafter fully described and particularly pointed out in the claims.
The following description and the annexed drawings set forth in detail certain illustrative
embodiments of the invention. These embodiments are indicative, however, of but a
few of the various ways in which the principles of the invention may be employed.
Other objects, advantages and novel features of the invention will become apparent
from the following detailed description of the invention when considered in conjunction
with the drawings.
BRIEF DESCRIPTION OF DRAWINGS
[0015]
- FIG. 1. shows a schematic representation of the layer structure of a conventional
infra-red Schottky barrier photodetector.
- FIG. 2. shows a simplified illustration of the electron density profile at three times
after a metal region is illuminated with light.
- FIG. 3. shows a schematic representation of the conduction band of a Schottky barrier
photodetector, how it extracts hot carriers and and how it responds to forward an
reverse bias
- FIG. 4. shows a graph demonstrating how increased selectivity of extraction (decreased
extraction energy width ΔE) increases efficiency of a hot carrier photovoltaic cell.
It then shows two examples of cells with low selectivity (standard Schottky barrier
structure) and high selectivity (tunneling structure).
- FIG. 5A and FIG. 5B. show a schematic representation of the layer structure of the
proposed photovoltaic device.
- FIG. 6. shows a schematic representation of the layer structure of the proposed photovoltaic
device with all contacts at the top and an optimised reflective back layer.
- FIG. 7. shows a schematic representation of the device realised in nanowire or nanofins
with an intervening matrix (706) to reduce the refractive index of the semiconductor
layer.
- FIG.8. shows a schematic representation of the conduction band of the proposed photovoltaic
device realised in GaAs/AlGaAs before and after deposition of a Chromium layer.
- FIG.9. shows experimental results of the current-voltage characteristic of the tunneling
device in comparison with a Schottky device, showing an order of magnitude increase
in current for the same illumination intensity (illumination at 785nm).
- FIG. 10 shows the normalised output current for a device such as that of FIG. 9 compared
with a standard Schottky device.
- FIG.11. shows a schematic illustration of the layer structure of a device realised
in AlInAs/InGaAs with a Chromium metallic absorber layer
- FIG.12. shows the reflection of the layer structure detailed in FIG. 11 as a function
of wavelength of illumination,
- FIG.13. shows a schematic representation of the conduction band (1300) and valence
band (1301) structure of the Cr/AlInAs/InGaAs structure
DETAILED DESCRIPTION OF THE INVENTION
Definition Of Terms
[0016]
- Ballistic carrier: a charge carrier (electron or hole) that has not suffered a scattering
event, with the associated change in energy and/or momentum. A photo-generated ballistic
carrier preserves information about the photon that generated it, in the form of excess
kinetic energy.
- Hot carrier: a carrier whose kinetic energy is larger than the average equilibrium
kinetic energy of the same type of carrier in a given material. Due to the relation
between kinetic energy and temperature, a hot carrier is defined as a carrier in a
population with a temperature higher than the lattice temperature. A carrier can suffer
a number of scattering events and still be a hot carrier, but would not be ballistic.
- Mean free path: the average length travelled by a carrier between scattering events.
It is possible to assign different mean free paths to different scattering mechanisms.
- Thermalisation: the process by which a carrier loses its excess energy. This process
is generally the combined effect of several scattering mechanisms.
[0017] (Technical problem) In previously presented hot carrier photovoltaic cells in the prior art, absorption
occurs through band-to-band absorption of light in a semiconductor. Because the photogenerated
carriers need to be extracted within a distance less than their mean free path, this
necessitates very thin layers for the semiconductor absorbers. This results in low
total light absorption (∼1%) in these layers. In previously presented Schottky barrier
infra-red photodetectors, photoexcited ballistic carriers are extracted when they
have energy in excess of the Schottky barrier height. This results in low device efficiency
as the energy held by carriers beneath the Schottky barrier height is lost.
[0018] (Technical solution) By absorbing light in a thin metallic film, the present invention enhances the absorption,
as metallic films as thin as 8nm can absorb over 99% of incident light over a broadband
from 400-1000nm as long as they have an appropriate refractive index, anti-reflective
coating and phase matching layer to allow resonant light absorption in the thin metallic
layer. This metallic absorption is currently used in Schottky barrier photodetectors,
but in these devices large amounts of energy is wasted as the extraction of carriers
does not occur energy selectively.
[0019] The present invention may include a semiconductor device in which light is first
absorbed in a metallic layer, generating a hot carrier distribution. These photoexcited
hot carriers are then extracted energy selectively. Any carriers which are not at
the correct energy to be selectively extracted are redistributed in energy by carrier-carrier
scattering, remain hotter than the lattice, and can still contribute to the overall
efficiency of this device.
[0020] An example of such a device and its efficiency in comparison with a weakly selective,
Schottky barrier, device is shown in FIG.4. The selectivity is defined by a parameter
ΔE, on the abscissa of graph (400). This parameter is defined as the energy width
for which the probability of transmission from the metal to the semiconductor region
is greater than 0.5. The ordinate axis of graph (400) has the efficiency of such a
device, so that we can see how selectivity of extraction impacts device efficiency.
[0021] Redistribution of carrier energy in a weakly selective Schottky barrier cell (402)
(i.e. extracting hot rather than ballistic carriers) increases the efficiency of such
a device from the theoretical 10-20% maximum for a ballistic Schottky photodetector
to 50-60% for a Schottky barrier device operating with hot carriers. This is due to
recirculation of the energy of carriers which are not transmitted over the barrier.
In reality this improvement is hard to achieve as it requires very fine tuning of
the absorber thickness, to be thicker than the electron-electron mean free path but
thinner than the electron-phonon mean free path, and no redistribution of energy from
the higher energy carriers is possible as these are all transmitted.
[0022] By increasing the selectivity of the extraction (reducing ΔE) the efficiency of the
device is increased to 85%, almost an order of magnitude higher efficiency than one
might expect to achieve with a standard ballistic Schottky barrier photodetector.
This ultra-selective extraction can be achieved, for example, by resonant tunneling
of carriers from the metal through a double barrier quantum well, the device structure
for which is schematically illustrated in 401. The semiconductor layers are necessarily
n-doped in order to give the band bending required to produce a Schottky barrier,
and they also have different conduction band energies in order to give rise to a quantum
well region for extraction. The implementation of these is explored in more detail
in various embodiments below.
[0023] In this device (401) light is absorbed in the metal layer and a cold population of
electrons (403) is warmed to become a hot population (404), electrons from the hot
population are then extracted energy selectively through a bound state in the quantum
well (405) at the resonant tunneling energy for which the transmission probability
of carriers from the metal to the semiconductor layers is substantially unity. For
all other energies the transmission probability is very low and reflection of carriers
at the metal/semiconductor interface occurs, followed by the carriers redistributing
their energy (in particular providing further electrons at the resonant tunneling
energy). This differs from the Schottky operation shown in (402) in which a cold distribution
(406) is warmed to a hot distribution (407) and from which all carriers in excess
of the Schottky barrier height can transfer (407) to the semiconductor layers.
[0024] In addition to improved device efficiency, extracting carriers through resonant tunneling
at an energy lower than the Schottky barrier height gives the additional benefit that
this device can operate at significantly longer wavelengths than in a traditional
Schottky barrier photodetector. This is because with a traditional Schottky barrier
photodetector it is necessary for electrons to be excited to energies in excess of
the Schottky barrier. However, with tunneling extraction, excitation only to the tunneling
energy is required, which will necessarily be lower than the Schottky barrier height.
[0025] More generally and in accordance with description already provided, technical features
of the present invention include the following:
- A thin metallic absorbing layer, thinner than the electron mean free path in the metal.
- An n-doped semiconductor adjacent to the metallic layer to provide extraction of electrons
from the metal.
- Energy selective extraction of carriers.
- A reflective back surface.
- An additional ohmic contact to the semiconductor layer.
[0026] Furthermore, the present invention may include the following additional features:
- Optimum thickness metallic layer.
- Optimum thickness semiconductor to provide optical broadband resonance.
- Selective extraction of carriers by resonant tunnelling.
- An optimised conduction band offset.
- Optimised Schottky barrier height between metal and semiconductor.
- Reflective back surface provided by a thick metallic layer.
- Reflective back surface provided by a Bragg reflector.
[0027] Aspects of the invention, therefore, include a semiconductor device having a layered
structure. In exemplary embodiments, the semiconductor device may include a metallic
layer of thickness 1-100nm, with a thickness optimised to absorb light in a wavelength
range of operation. The device further may include an adjacent semiconductor layer
additionally adjacent to an ohmic electrical contact, wherein the interface between
the metallic layer and the semiconductor layer is electrically rectifying and energy
selective. The device further may include a reflective back surface positioned opposite
to the semiconductor layer relative to incident light providing broadband reflection
in the wavelength range of operation. The semiconductor layer may include a quantum
well adjacent to the metallic layer, wherein the energy selectivity is provided by
the quantum well allowing charge carrier tunneling from the metallic layer. The device
further may include an additional anti-reflection dielectric layer deposited on the
metallic layer that is configured to minimise reflection of light in the wavelength
range of operation.
[0028] The semiconductor device described herewith may be physically implemented in numerous
embodiments, detailed in the following section, but it is to be understood that there
may be many other ways in which the device with the structure presented above may
be built, and are implicitly incorporated in this disclosure.
Embodiment 1
[0029] FIG. 5A presents a possible architecture of the energy selective metallic photovoltaic
device. In this schematic a thin metallic layer (501) (thinner than the electron mean
free path in the chosen metal) is disposed on a series of semiconductor layers, which
will give the band structure as illustrated in FIG.4 (401). The metallic layer (501)
may have a thickness of 1-100 nm and may be optimized to absorb light in the wavelength
range of operation. The semiconductor layers (502, 503, 504), which may include a
quantum well layer (503) in between two semiconductor barrier layers (502) and (504)
should be n-doped and the semiconductor barrier layers (502, 504) should have a conduction
band which has a positive potential energy offset from the quantum well layer (503).
This can be achieved, among other ways, by having the semiconductor barrier material
being of a wider band gap energy than the semiconductor well layer. In this way a
quantum well is formed inside a Schottky barrier, whereby transmission of carriers
from the metallic layer (501) into the semiconductor barrier material (504) is only
possible at discrete energies substantially equal to the bound state energies of the
quantum well.
[0030] There are various exemplary configurations for different uses of the described device
structure. For example, the semiconductor device may be configured to extract carriers
against an external resistance so as to convert optical energy in to electrical energy.
The semiconductor device further may be associated with an external bias voltage source
that applies an external bias, wherein carriers are extracted with the assistance
of the external bias so as to operate the semiconductor device as a photodetector.
Embodiment 2.
[0031] In a further embodiment, the light absorption in the structure is enhanced so that
light is resonantly absorbed in the thin metallic layer or layers. Resonant absorption
is achieved when the thicknesses of the anti-reflective layer, metal layer and semiconductor
layer (or layers) are optimised to ensure light reflecting off the metal absorber
layer interferes destructively with light reflecting off the back surface reflector.
In practice this can be achieved by choosing the layer thickness of the antireflective
coating (507) and the series of semiconductor layers (508) prior to the reflective
contact to be approximately equal to one quarter of the wavelength of light in these
regions respectively. For a broadband source of light this value is taken as the central
wavelength of light in a spectrum or computationally optimised.
[0032] In addition to this relationship for the anti-reflective coating layer and semiconductor
layers, the thickness of the metal can also be optimised so that an equal amount of
light is reflected from the metal layer (501) as is transmitted through it and will
be reflected off the back surface reflector (505). In this way the destructive interference
of light is optimised and the resonant absorption in the metal layer can be maximised.
[0033] A specific embodiment in Chromium and GaAs/AlGaAs showing optimum thicknesses required
to achieve high absorption is shown in FIG. 6. In this structure each of the ARC layers
(600) and semiconductor layers (602, 603,604) has been computationally optimised to
achieve maximum absorption in the Cr layer (601), with a fixed thickness back reflector
of Pd (605). The graph in FIG 6 (606) shows the absorption as a function of Cr thickness,
showing that optimum absorption over 90% is achieved for a Cr thickness of 15nm and
decreases for thicker layers. This thickness of 15nm is also below the electron mean
free path in Cr and so is an electrically and optically optimum thickness for this
device.
[0034] Generally, therefore, a thickness of the anti-reflection layer and a thickness of
the semiconductor layer may be optimised such that light reflecting off each of the
anti-reflection layer and the semiconductor layer destructively interferes. In addition,
the thickness of the antireflection layer and a thickness of the semiconductor layer
may be substantially equal to one quarter of the wavelength of light in those layers.
Furthermore, the metallic layer thickness may be optimised such that in the absence
of any other layers the metallic layer would transmit 50% of the incident light in
the wavelength range of operation. The thickness of the metallic layer also may be
less than a mean free path of a photoexcited electron in the metallic layer in the
wavelength range of operation.
[0035] In addition to this thickness optimisation of the metal and anti-reflection layers,
there can also be structural optimisation to allow plasmonic absorption in the metal.
This can be achieved, among many other ways, by creating a grating or prism structure
to enhance the electric field perpendicular to the metal/semiconductor interface.
Such a structure is schematically represented in FIG. 5B, in which the metal layer
has a grating structure (509) to improve absorption. The distance between adjacent
metallic features (510) of the grating can be optimised to allow strong absorption
for particular wavelengths of illumination, but is not generally able to be optimised
for broadband absorption, so only finds use in wavelength specific embodiments of
this invention rather than broadband embodiments.
Embodiment 3.
[0036] In a further embodiment of this device the metallic layer is chosen to have a both
a refractive index and extinction coefficient of between 2 and 5, or more preferably
between 3 and 4 over the wavelength range of interest. Choosing a metal with these
properties acts to enhance Drude absorption and minimise the reflection of light from
the metal surface.
[0037] A particular example of this is Chromium for the wavelength range of 400-1000nm.
Other metals which also have this property are Molybdenum, Bismuth and Cobalt, and
this property could be achieved more widely (and tuneably) with suitable combinations
of metallic layers in an alloy (e.g. a Nickel/Cobalt alloy).
Embodiment 4.
[0038] In a further embodiment shown in FIG.7, the device can be contacted entirely from
the top, by etching down through antireflective coating layer (700), metal layer (701)
semiconductor barrier layer (702) and semiconductor quantum well layer (703), which
may have properties similar to previous embodiments, to the semiconductor barrier
layer (704) and forming an ohmic contact (707) to the semiconductor layer from the
top side. This allows full flexibility in choosing the back reflective layer (705)
as it no longer needs to be an ohmic contact layer as well. In this case the reflective
layer might be a particularly reflective metal, such as silver, or even a stack of
semiconductor layers tuned to provide optimised reflection over a broad or narrow
range of wavelengths or a (such as a distributed Bragg reflector (DBR)).
Embodiment 5.
[0039] In order to allow a thicker semiconductor series of layers, a nanowire or nanofin
construction may be used as schematically illustrated in FIG. 8. Similar to previous
embodiments, the embodiment of Fig. 8 may include an antireflective coating layer
(800), metal layer (801) semiconductor barrier layer (802), semiconductor quantum
well layer (803), another semiconductor 804, and a back reflective layer (805) which
may have properties similar to previous embodiments
[0040] In such an arrangement of Fig. 8 the semiconductor layer structure is grown or etched
as a series of nanowires (one dimensional) or nanofins (two dimensional) with voids
in between in a plane perpendicular to the direction of incident light. These voids
(806) can be filled with a dielectric medium and allow a thicker semiconductor layer
series, as the effective refractive index of this composite layer will be the weighted
average of the semiconductor layers and the interleaving dielectric (806). This average
refractive index can be lower than the refractive index of the semiconductor layer
series, and therefore the optimised thickness of this composite layer, equal to one
quarter of a wavelength of incident light in that material, can be thicker.
[0041] In addition to being thicker, having a different refractive index in these layers
allows a refractive index change between semiconductor barrier layer (804) and the
back reflective layer (805). This can allow layers (804) and (805) to be grown in
the same materials and still provide a reflective interface.
Embodiment 6.
[0042] A specific materials system of interest is GaAs/AIGaAs, owing to mature processing
technologies and design capabilities available for such materials. The layer thicknesses
required to provide optimum optical absorption have already been calculated for this
materials system with a Cr cap in FIG. 6 However, the electronic properties of the
semiconductor well can also be improved to allow improved extraction of electrons
from the metal layer(s).
[0043] A design for the semiconductor layer structure, shown as the conduction band profile
(902) and (903), of such a device is shown in FIG. 9, respectively both before (900)
and after (901) Chromium deposition.
[0044] The device in the example of FIG. 9 has a 4nm Al
0.4Ga
0.6As barrier, followed by a 15nm graded Al
xGa
1-xAs well (with x=0 at the metal side and x=0.3 at the rear side), followed by a 100nm
layer of Al
0.4Ga
0.6As. All these layers are doped to 5x10
17 to give a Schottky barrier when a Chromium layer of 22nm is deposited. The well is
graded so that the conduction band structure is substantially flat when electrons
are extracted, allowing better extraction than with a pure GaAs well.
[0045] The normalised output current for such a device compared with a standard Schottky
device (realised in n-Al
0.4Ga
0.6As with the same thickness Cr layer) is shown in FIG.10, revealing an order of magnitude
increase in current for the tunneling extraction structure (1001 and 1003) compared
with the Schottky extraction structure (1002 and 1004).
Embodiment 7.
[0046] A further embodiment of the device in a particular materials system is shown in FIG.
11, in the InGaAs/AlInAs materials system. This and the AlGaAs/GaAs embodiment are
two example materials systems, but many other semiconductor materials systems exist
for which similar band structures could be created, such as but not limited to InGaAs/GaAs/AlGaAs,
InGaSb/GaSb/AlGaSb, GaInP/GaAs, InGaN/GaN/AlGaN, Si/Ge/Sn and InGaAs/InAlAs/InP.
[0047] In FIG.11, an optimised layer stack of an SiO2 anti-reflection coating (1100) is
disposed on a Chromium metallic absorber (1101) with tunneling extraction provided
by a series of layers of nAlInAs (1102), nInGaAs (1103), and nAlInAs(1104) on a gold
reflector and contact (1105). This stack has the reflection profile shown in FIG 12,
which shows the reflection of the layer structure detailed in FIG. 11 as a function
of wavelength of illumination, whereby the absorption into the Cr layer is greater
than 90% across this wavelength range.
[0048] FIG.13. shows the conduction band (1300) and valence band (1301) profiles of this
structure showing its compatability with the device operation already proposed for
the tunneling extraction photovoltaic or photodetector device.
[0049] As aspect of the invention is a semiconductor device with a layer structure. In exemplary
embodiments, the semiconductor device may include a metallic layer of a thickness
1-100nm, with a thickness optimised to absorb light in a wavelength range of operation;
an adjacent semiconductor layer additionally adjacent to an ohmic electrical contact,
wherein an interface between the metallic layer and the semiconductor layer is electrically
rectifying and energy selective; and the semiconductor layer includes a quantum well
adjacent to the metallic layer, wherein the energy selectivity is provided by the
quantum well allowing charge carrier tunneling from the metallic layer. Embodiments
of the semiconductor device may include one or more of the following features, either
individually or in combination.
[0050] In an exemplary embodiment of the semiconductor device, the semiconductor device
further may include a reflective back surface positioned opposite to the semiconductor
layer relative to incident light providing broadband reflection in the wavelength
range of operation.
[0051] In an exemplary embodiment of the semiconductor device, an additional anti-reflection
dielectric layer is deposited on the metallic layer, and the anti-reflection dielectric
layer is configured to minimise reflection of light in the wavelength range of operation.
[0052] In an exemplary embodiment of the semiconductor device, a thickness of the anti-reflection
dielectric layer and a thickness of the semiconductor layer are optimised such that
light reflecting off each of the anti-reflection dielectric layer and the semiconductor
layer destructively interferes.
[0053] In an exemplary embodiment of the semiconductor device, the thickness of the antireflection
dielectric layer and the thickness of the semiconductor layer is substantially equal
to one quarter of the wavelength of light in those layers.
[0054] In an exemplary embodiment of the semiconductor device, the metallic layer thickness
is optimised such that in the absence of any other layers the metallic layer would
transmit 50% of the incident light in the wavelength range of operation.
[0055] In an exemplary embodiment of the semiconductor device, a thickness of the metallic
layer is less than a mean free path of a photoexcited electron in the metallic layer
in the wavelength range of operation.
[0056] In an exemplary embodiment of the semiconductor device, the metallic layer acts as
a front electrical contact.
[0057] In an exemplary embodiment of the semiconductor device, the metallic layer has a
grating configured for enhancing plasmonic absorption.
[0058] In an exemplary embodiment of the semiconductor device, the energy selectivity is
such that the transmission of electrons from the metallic layer occurs over an energy
width of less than 0.5eV.
[0059] In an exemplary embodiment of the semiconductor device, a refractive index and extinction
coefficient of the metallic layer is between 3-4 in the wavelength range of operation.
[0060] In an exemplary embodiment of the semiconductor device, the metallic layer is any
one of Chromium, Bismuth or Molybdenum.
[0061] In an exemplary embodiment of the semiconductor device, the metallic layer comprises
a plurality of metal layers including a first metallic layer deposited on the semiconductor
layer.
[0062] In an exemplary embodiment of the semiconductor device, the anti-reflection dielectric
layer comprises a plurality of anti-reflection coating layers.
[0063] In an exemplary embodiment of the semiconductor device, the semiconductor layer comprises
a plurality of layers and one of the layers is the quantum well.
[0064] In an exemplary embodiment of the semiconductor device, the semiconductor layer is
grown or etched to be configured as nanowires or nanofins in a direction perpendicular
to a growth direction with a dielectric medium disposed between adjacent nanowires
or nanofins.
[0065] In an exemplary embodiment of the semiconductor device, the quantum well is provided
by a GaAs layer disposed between two AlGaAs layers.
[0066] In an exemplary embodiment of the semiconductor device, the quantum well is provided
by an InGaAs layer disposed between two AlInAs layers.
[0067] In an exemplary embodiment of the semiconductor device, the semiconductor device
is configured to extract carriers against an external resistance so as to convert
optical energy in to electrical energy.
[0068] In an exemplary embodiment of the semiconductor device, the semiconductor device
further may include a bias voltage source that applies an external bias, wherein carriers
are extracted with the assistance of the external bias so as to operate the semiconductor
device as a photodetector.
[0069] Although the invention has been shown and described with respect to a certain embodiment
or embodiments, equivalent alterations and modifications may occur to others skilled
in the art upon the reading and understanding of this specification and the annexed
drawings. In particular regard to the various functions performed by the above described
elements (components, assemblies, devices, compositions, etc.), the terms (including
a reference to a "means") used to describe such elements are intended to correspond,
unless otherwise indicated, to any element which performs the specified function of
the described element (i.e., that is functionally equivalent), even though not structurally
equivalent to the disclosed structure which performs the function in the herein exemplary
embodiment or embodiments of the invention. In addition, while a particular feature
of the invention may have been described above with respect to only one or more of
several embodiments, such feature may be combined with one or more other features
of the other embodiments, as may be desired and advantageous for any given or particular
application.
INDUSTRIAL APPLICABILITY
[0070] The invention finds application in photodetection and energy conversion. This device
may be used as an efficient photodetector for a very wide range of wavelengths, tuned
by altering the tunneling energy. It may also be used as an efficient photovoltaic
cell, converting broadband incident light into electrical energy. Owing to its particular
novelty at detecting and converting light with long wavelengths, this device may also
be used as an efficient photovoltaic cell in a thermophotovoltaic set up.
1. A semiconductor device with a layer structure comprising:
a metallic layer of a thickness 1-100nm, with a thickness optimised to absorb light
in a wavelength range of operation;
an adjacent semiconductor layer additionally adjacent to an ohmic electrical contact,
wherein an interface between the metallic layer and the semiconductor layer is electrically
rectifying and energy selective; and
the semiconductor layer includes a quantum well adjacent to the metallic layer, wherein
the energy selectivity is provided by the quantum well allowing charge carrier tunneling
from the metallic layer.
2. The semiconductor device of claim 1, further comprising a reflective back surface
positioned opposite to the semiconductor layer relative to incident light providing
broadband reflection in the wavelength range of operation.
3. The semiconductor device of any of claims 1-2, wherein an additional anti-reflection
dielectric layer is deposited on the metallic layer, and the anti-reflection dielectric
layer is configured to minimise reflection of light in the wavelength range of operation.
4. The semiconductor device of claim 3, wherein a thickness of the anti-reflection dielectric
layer and a thickness of the semiconductor layer are optimised such that light reflecting
off each of the anti-reflection dielectric layer and the semiconductor layer destructively
interferes.
5. The semiconductor device of claim 4, wherein the thickness of the antireflection dielectric
layer and the thickness of the semiconductor layer is substantially equal to one quarter
of the wavelength of light in those layers.
6. The semiconductor device of any of claims 1-5, wherein the metallic layer thickness
is optimised such that in the absence of any other layers the metallic layer would
transmit 50% of the incident light in the wavelength range of operation.
7. The semiconductor device of any of claims 1-6, wherein a thickness of the metallic
layer is less than a mean free path of a photoexcited electron in the metallic layer
in the wavelength range of operation.
8. The semiconductor device of any of claims 1-7, wherein the metallic layer acts as
a front electrical contact.
9. The semiconductor device of any of claims 1-8, wherein the metallic layer has a grating
configured for enhancing plasmonic absorption.
10. The semiconductor device of any of claims 1-9, wherein the energy selectivity is such
that the transmission of electrons from the metallic layer occurs over an energy width
of less than 0.5eV.
11. The semiconductor device of any of claims 1-10, wherein a refractive index and extinction
coefficient of the metallic layer is between 3-4 in the wavelength range of operation.
12. The semiconductor device of any of claims 1-11, wherein the metallic layer is any
one of Chromium, Bismuth or Molybdenum.
13. The semiconductor device of any of claims 1-12, wherein the metallic layer comprises
a plurality of metal layers including a first metallic layer deposited on the semiconductor
layer.
14. The semiconductor device of any of claims 1-13, wherein the anti-reflection dielectric
layer comprises a plurality of anti-reflection coating layers.
15. The semiconductor device of any of claims 1-14, wherein the semiconductor layer comprises
a plurality of layers and one of the layers is the quantum well.
16. The semiconductor device of any of claims 1-15, wherein the semiconductor layer is
grown or etched to be configured as nanowires or nanofins in a direction perpendicular
to a growth direction with a dielectric medium disposed between adjacent nanowires
or nanofins.
17. The semiconductor device of any of claims 1-16, wherein the quantum well is provided
by a GaAs layer disposed between two AIGaAs layers.
18. The semiconductor device of any of claims 1-16, wherein the quantum well is provided
by an InGaAs layer disposed between two AllnAs layers.
19. The semiconductor device of any of claims 1-18, wherein the semiconductor device is
configured to extract carriers against an external resistance so as to convert optical
energy in to electrical energy.
20. The semiconductor device of any of claims 1-18, further comprising a bias voltage
source that applies an external bias, wherein carriers are extracted with the assistance
of the external bias so as to operate the semiconductor device as a photodetector.
1. Halbleitervorrichtung mit einer Schichtstruktur, umfassend:
eine Metallschicht mit einer Dicke von 1 - 100 nm, wobei eine Dicke optimiert ist,
um Licht in einem Wellenlängen-Arbeitsbereich zu absorbieren;
eine benachbarte Halbleiterschicht, die zusätzlich einem ohmschen elektrischen Kontakt
benachbart ist, wobei eine Grenzfläche zwischen der Metallschicht und der Halbleiterschicht
elektrisch gleichrichtend und energieselektiv ist; und
die Halbleiterschicht einen Quantentopf der Metallschicht benachbart enthält, wobei
die Energieselektivität durch den Quantentopf geliefert wird, der ein Ladungsträgertunneln
aus der Metallschicht erlaubt.
2. Halbleitervorrichtung nach Anspruch 1, ferner umfassend eine in Bezug auf ein einfallendes
Licht der Halbleiterschicht entgegengesetzt positionierte reflektierende rückwärtige
Oberfläche, die eine breitbandige Reflexion im Wellenlängen-Arbeitsbereich liefert.
3. Halbleitervorrichtung nach einem der Ansprüche 1 - 2, wobei eine zusätzliche dielektrische
Antireflexionsschicht auf der Metallschicht abgeschieden ist und die dielektrische
Antireflexionsschicht dafür konfiguriert ist, eine Reflexion von Licht im Wellenlängen-Arbeitsbereich
zu minimieren.
4. Halbleitervorrichtung nach Anspruch 3, wobei eine Dicke der dielektrischen Antireflexionsschicht
und eine Dicke der Halbleiterschicht so optimiert sind, dass von jeder der dielektrischen
Antireflexionsschicht und der Halbleiterschicht reflektierendes Licht destruktiv interferiert.
5. Halbleitervorrichtung nach Anspruch 4, wobei die Dicke der dielektrischen Antireflexionsschicht
und die Dicke der Halbleiterschicht im Wesentlichen gleich einer Viertel-Wellenlänge
von Licht in jenen Schichten ist.
6. Halbleitervorrichtung nach einem der Ansprüche 1 - 5, wobei die Metallschichtdicke
so optimiert ist, dass bei Fehlen jeglicher anderer Schichten die Metallschicht 50
% des einfallenden Lichts im Wellenlängen-Arbeitsbereich durchlassen würde.
7. Halbleitervorrichtung nach einem der Ansprüche 1 - 6, wobei eine Dicke der Metallschicht
geringer als eine mittlere freie Weglänge eines fotoangeregten Elektrons in der Metallschicht
im Wellenlängen-Arbeitsbereich ist.
8. Halbleitervorrichtung nach einem der Ansprüche 1 - 7, wobei die Metallschicht als
ein vorderseitiger elektrischer Kontakt dient.
9. Halbleitervorrichtung nach einem der Ansprüche 1 - 8, wobei die Metallschicht ein
Gitter aufweist, das zum Steigern einer Plasmonenabsorption konfiguriert ist.
10. Halbleitervorrichtung nach einem der Ansprüche 1 - 9, wobei die Energieselektivität
derart ist, dass die Transmission von Elektronen aus der Metallschicht über eine Energiebreite
von weniger als 0,5 eV auftritt.
11. Halbleitervorrichtung nach einem der Ansprüche 1 - 10, wobei ein Brechungsindex und
auf Extinktionskoeffizient der Metallschicht im Wellenlängen-Arbeitsbereich zwischen
3 und 4 liegt.
12. Halbleitervorrichtung nach einem der Ansprüche 1 - 11, wobei die Metallschicht eine
aus Chrom, Wismut oder Molybdän ist.
13. Halbleitervorrichtung nach einem der Ansprüche 1 - 12, wobei die Metallschicht eine
Vielzahl von Metallschichten umfasst, die eine auf der Halbleiterschicht abgeschiedene
erste Metallschicht einschließen.
14. Halbleitervorrichtung nach einem der Ansprüche 1 - 13, wobei die dielektrische Antireflexionsschicht
eine Vielzahl dielektrischer Antireflexions-Deckschichten umfasst.
15. Halbleitervorrichtung nach einem der Ansprüche 1 - 14, wobei die Halbleiterschicht
eine Vielzahl von Schichten umfasst und eine der Schichten der Quantentopf ist.
16. Halbleitervorrichtung nach einem der Ansprüche 1 - 15, wobei die Halbleiterschicht
so aufgewachsen oder geätzt ist, dass sie als Nanodrähte oder Nanolamellen in einer
Richtung senkrecht zu einer Wachstumsrichtung konfiguriert sind, wobei ein dielektrisches
Medium zwischen benachbarten Nanodrähten oder Nanolamellen angeordnet ist.
17. Halbleitervorrichtung nach einem der Ansprüche 1 - 16, wobei der Quantentopf durch
eine zwischen zwei AlGaAs-Schichten angeordnete GaAs-Schicht vorgesehen ist.
18. Halbleitervorrichtung nach einem der Ansprüche 1 - 16, wobei der Quantentopf durch
eine zwischen zwei AlInAs-Schichten angeordnete InGaAs-Schicht vorgesehen ist.
19. Halbleitervorrichtung nach einem der Ansprüche 1 - 18, wobei die Halbleitervorrichtung
dafür konfiguriert ist, Träger gegen einen externen Widerstand zu extrahieren, um
optische Energie in elektrische Energie umzuwandeln.
20. Halbleitervorrichtung nach einem der Ansprüche 1 - 18, ferner umfassend eine Vorspannungsquelle,
die eine externe Vorspannung anlegt, wobei Träger mit der Unterstützung der Vorspannung
extrahiert werden, um die Halbleitervorrichtung als Fotodetektor zu betreiben.
1. Dispositif à semiconducteur avec une structure en couches comprenant :
une couche métallique d'une épaisseur de 1-100 nm, avec une épaisseur optimisée pour
absorber la lumière dans une plage de longueur d'onde de fonctionnement ;
une couche de semiconducteur adjacente, adjacente en supplément à un contact électrique
ohmique, dans lequel une interface entre la couche métallique et la couche de semiconducteur
est électriquement redresseuse et sélective en énergie ; et
la couche de semiconducteur inclut un puits quantique adjacent à la couche métallique,
dans lequel la sélectivité d'énergie est fournie par le puits quantique permettant
un franchissement par effet tunnel de porteur de charge depuis la couche métallique.
2. Dispositif à semiconducteur de la revendication 1, comprenant également une surface
arrière réfléchissante positionnée à l'opposé de la couche de semiconducteur par rapport
à la lumière incidente, offrant une réflexion à large bande dans la plage de longueur
d'onde de fonctionnement.
3. Dispositif à semiconducteur de l'une quelconque des revendications 1 à 2, dans lequel
une couche diélectrique antiréfléchissante supplémentaire est déposée sur la couche
métallique, et la couche diélectrique antiréfléchissante est configurée pour minimaliser
la réflexion de la lumière dans la plage de longueur d'onde de fonctionnement.
4. Dispositif à semiconducteur de la revendication 3, dans lequel une épaisseur de la
couche diélectrique antiréfléchissante et une épaisseur de la couche de semiconducteur
sont optimisées de telle sorte que la réflexion de la lumière sur chaque couche diélectrique
antiréfléchissante et la couche de semiconducteur interfèrent de manière destructive.
5. Dispositif à semiconducteur de la revendication 4, dans lequel l'épaisseur de la couche
diélectrique antiréfléchissante et l'épaisseur de la couche de semiconducteur sont
sensiblement égales à un quart de la longueur d'onde de la lumière dans ces couches.
6. Dispositif à semiconducteur de l'une quelconque des revendications 1 à 5, dans lequel
l'épaisseur de couche métallique est optimisée de telle sorte qu'en l'absence d'autres
couches la couche métallique transmettrait 50 % de la lumière incidente dans la plage
de longueur d'onde de fonctionnement.
7. Dispositif à semiconducteur de l'une quelconque des revendications 1 à 6, dans lequel
une épaisseur de la couche métallique est inférieure au libre parcours moyen d'un
électron photo-excité dans la couche métallique dans la plage de longueur d'onde de
fonctionnement.
8. Dispositif à semiconducteur de l'une quelconque des revendications 1 à 7, dans lequel
la couche métallique agit comme un contact électrique avant.
9. Dispositif à semiconducteur de l'une quelconque des revendications 1 à 8, dans lequel
la couche métallique a une grille pour accroître l'absorption plasmonique.
10. Dispositif à semiconducteur de l'une quelconque des revendications 1 à 9, dans lequel
la sélectivité d'énergie est telle que la transmission d'électrons depuis la couche
métallique se produit sur une largeur d'énergie de moins de 0,5 eV.
11. Dispositif à semiconducteur de l'une quelconque des revendications 1 à 10, dans lequel
un indice de réfraction et un coefficient d'extinction de la couche métallique sont
situés entre 3 et 4 dans la plage de longueur d'onde de fonctionnement.
12. Dispositif à semiconducteur de l'une quelconque des revendications 1 à 11, dans lequel
la couche métallique est au moins en chrome, bismuth ou molybdène.
13. Dispositif à semiconducteur de l'une quelconque des revendications 1 à 12, dans lequel
la couche métallique comprend une pluralité de couches de métal incluant une première
couche métallique déposée sur la couche de semiconducteur.
14. Dispositif à semiconducteur de l'une quelconque des revendications 1 à 13, dans lequel
la couche diélectrique antiréfléchissante comprend une pluralité de couches de revêtement
antiréfléchissantes.
15. Dispositif à semiconducteur de l'une quelconque des revendications 1 à 14, dans lequel
la couche de semiconducteur comprend une pluralité de couches, et l'une des couches
est le puits quantique.
16. Dispositif à semiconducteur de l'une quelconque des revendications 1 à 15, dans lequel
la couche de semiconducteur est formée par croissance ou attaque chimique pour être
configurée comme des nanofils ou nano-ailettes dans une direction perpendiculaire
à une direction de croissance, avec un milieu diélectrique disposé entre des nanofils
ou nano-ailettes adjacents.
17. Dispositif à semiconducteur de l'une quelconque des revendications 1 à 16, dans lequel
le puits quantique est formé par une couche GaAs entre deux couches AlGaAs.
18. Dispositif à semiconducteur de l'une quelconque des revendications 1 à 16, dans lequel
le puits quantique est formé par une couche InGaAs disposée entre deux couches AlInAs.
19. Dispositif à semiconducteur de l'une quelconque des revendications 1 à 18, dans lequel
le dispositif semiconducteur est configuré pour extraire des porteuses par rapport
à une résistance externe de manière à convertir de l'énergie optique en énergie électrique.
20. Dispositif à semiconducteur de l'une quelconque des revendications 1 à 18, comprenant
également une source de tension de polarisation qui applique une tension externe,
dans lequel des porteurs sont extraits avec l'assistance de la tension externe de
manière à faire fonctionner le dispositif à semiconducteur comme photodétecteur.